1. Field of the Invention
The invention relates to the curving of glass sheets available on a horizontal or approximately horizontal support, for example, a conveyer, said curving being carried out by application of the sheets, brought to their softening temperature, against an upper form, then possibly through deposit of said sheets under the effect of gravity onto a lower form and after their deposit onto said lower form, said lower form being generally composed of a ring with an open curved profile at its center, further called a curving frame or skeleton.
2. Discussion of Background
It is known from European patent No. 5 306 to pass sheets of glass from their horizaontal or approximately horizontal support, for example their conveyer, to the upper form, using a current of gas under pressure blown under the sheets of glass, vertically from below upwards. This ascending gas current applies the sheets of glass against the upper form with a certain violence resulting in certain detrimental marks on the surface of the glass.
In effect, since the upper form is curved and the sheets of glass are initially flat, the first contact takes place on only a reduced portion of the surface, at the center in the large majority of cases, that is, those cases where the upper form is convex, and the marks are all the greater when the contact is more violent and when the contact surface is reduced.
It is further known from French patent document published under the No. 2 085 464 and from French patent application No. 8604962 to apply sheets of glass against the upper form using suction exerted at the periphery of the curving form, with said curving form being associated with a suction box. This method of applying sheets of glass overcomes the disadvantage of marking at the center of the sheets of glass. In effect, the walls of the suction box which surround the curving form by providing a space between said walls and said curving form in order to allow the suction, assist in the confinement of the air suction to the edges of the sheets of glass. The result is that the glass comes gently up against the upper curving form and hence results in better surface quality of the glass curved in this manner.
This technique is satisfactory for manufacturing glasses with conventional curvatures. However, for large curvatures of the upper form which lead to strongly curved glasses, it can be noted that, after the sheets of glass come against the curving form, there is a less tight application of said sheets against the curving form in certain areas, particularly at the center, which can go as far as detachment from these areas and in particular from said center of the sheets with regard to the curving form.